1. Field of the Disclosure
The disclosure relates generally to power monitoring techniques for optical transmitters and, more particularly, to photodiode arrays operating in voltage mode and with an integrated reference photodiode.
2. Brief Description of Related Technology
Many communication systems transmit massive amounts of data through an optical networking infrastructure, such as that used by local and long-distance carriers, and Internet service providers. Recently, content providers and other companies have begun building their own internal, optical networking infrastructure.
Some optical networks use optical transceivers that allow for simultaneous and direct communication between users. These transceivers commonly include an optical transmit portion that converts an electrical signal into a modulated light beam that is coupled to one end of an optical fiber, and a receive portion that receives an optical signal from the other end of the optical fiber and converts it into an electrical signal. Parallel optical transceivers are a class of such devices that allow for simultaneous data transmission over an array of optical fibers in parallel. The typical parallel optical transceiver has a semiconductor laser array (e.g., a vertical cavity surface emitter laser (VCSEL) array) for transmission and PIN photodiode array for reception, where a parallel optical ribbon fiber, inserted into the optical transceiver, couples to either the VCSEL array or the PIN diode array.
It is commonly known that optical transmitters require constant or near constant monitoring of power levels. VCSELs, like other laser sources, are susceptible to natural performance degradation over a lifecycle (e.g., a reduction in optical power for a given drive current/voltage), and temperature-dependent operation. These susceptibilities can produce unwanted variations in power levels, and thus monitoring is desired.
To compensate for power level fluctuations, optical transceivers often use power monitoring systems comprising photodiodes that measure light intensity from the output of the VCSELs, for example, from a portion of a VCSEL's modulated output light beam. The photodiode array may be formed as part of a power control loop in a closed loop configuration, typically implemented in an all digital configuration with a digital readout or other scanning circuit that scans intensity data and provides it to a VCSEL drive circuit. In some implementations, all such functionality is implemented in a signal processing circuit on the optical transceiver.
The conventional way of optical power monitoring in an optical transceiver is to convert the photocurrent from the monitoring photodiodes to a voltage, which is then measured through the ND converter of a microprocessor in the optical transceiver. At the simplest end, the current-to-voltage conversion is achieved using a resistor at each photodiode. Other techniques use more complex unit cell circuitry (i.e., circuitry repeated at each photodiode), including trans-impedance amplifiers and buffers. Including additional resistors or other, more complex unit cell circuits presents a substantial problem to designers of small form factor (SFF) optical transceivers, where space is very limited. Moreover, the trans-impedance gain of the current-to-voltage conversion circuitry must be careful chosen, with a maximum value to avoid saturation and a minimum value that avoids quantization errors in the A/D converter of the microprocessor. There is a trade off between performance and long control loop scan times and the voltage applied across each PIN diode.